Method and device for operating electronic ballasts for high intensity discharge (HID) lamps

ABSTRACT

A method and device for operating electronic ballasts for high intensity discharge lamps, the ballasts having a driver, two power switches, an LC series circuit, a driver controller, a current sensor, and a power sensor. The method includes the steps of (a) generating pulses of frequency f1 for a time t1, equal to n/f1, where f1 equals the LC resonance frequency; (b) monitoring the existence of current and (c) monitoring the current in the lamp circuit, and proceeding to (h) upon determining that there is no current in the lamp circuit, (d) continuing pulse generation for a time t2; (e) switching the frequency f2, at which a set power is reached; (f) monitoring and stabilizing the lamp power by modifying f2, and proceeding to step (h) when the set power is exceeded; (g) monitoring current and power according to steps (c) and (f); h) inhibiting pulse generation for a time approximately equal to t2/k; (i) proceeding to step (a) until t2 has elapsed; and (j) inhibiting pulse generation until power is switched off and on.

TECHNICAL FIELD

The present invention relates to the field of electronic solid stateballasts for High Intensity Discharge (HID) lamps, and morespecifically, it relates to a method and device utilizing solid stateballasts for operating HID lamps, e.g., High Pressure Sodium (HPS)lamps.

BACKGROUND ART

The term “discharge lamp” refers to a lamp in which the electric energyis transformed into optical radiation energy when electric current ispassed through a gas, metal vapor, or a mixture thereof, present insidethe lamp.

Presently, various circuits of electronic ballasts for discharge lamps,and in particular for fluorescent lamps, are known in the art. Aspecific example is the circuit shown in FIG. 1, which uses two powerswitches PS₁ and PS₂ in a totem pole (half-bridge) topology, the tubecircuit consisting of an L-C series resonant circuit. The power switchesrepresented by power MOSFETS are driven to alternatively conduct, e.g.,by a MOS Gate Driver (IR2155)(MGD). The MGD provides a high frequency(20 to 80 kHz) square wave output, with the frequency of oscillationgiven by: $f_{osc} = {\frac{1}{1.4R_{T}C_{T}}.}$

Prior to striking the fluorescent lamp 2, the resonant circuit consistsof L, C₁ and C₂ connected in series. Since C₂ has a lower value than C₁,it operates at a higher AC voltage than the latter, and in fact, it isthis higher voltage that strikes the lamp. After the lamp strikes, C₂ iseffectively shorted by the lamp voltage drop, and the resonant frequencyof the circuit is now determined by L and C₁.

Under resonance conditions, the sinusoidal voltage across the lamp isamplified by a factor of Q (Q being the circuit quality factor) and theamplitude of this voltage attains a value sufficient for striking thelamp, which thereafter gives a non-blinking light.

The above-described basic circuit is well-suited for fluorescent lamps,but will not adequately work for arc discharge lamps or HID lamps.

Initially, the HID lamp is an open circuit. Short pulses of voltagesuffice to strike the lamp, provided the pulses are of adequateamplitude (about 4,500 Volts). Subsequent to striking, the resistance ofthe lamp drops drastically and then slowly rises to its normal operatinglevel. Hence, to prevent lamp damage subsequent to striking and duringthe warm-up, the current of the lamp must be restricted.

It is a characteristic of HID lamps that their voltage increases overthe life of the lamp, due to a slow increase of stabilizationtemperature. Therefore, unless the lamp ballast maintains the lamppower, the light output of the lamp will vary to an unacceptable degree.

Ballast devices for HD lamps should be different from ballasts forfluorescent lamps, for the following main reasons:

1) these devices should withstand open-circuit operation conditions;

2) they should supply sufficiently high power for striking the lamp at avoltage of 3 to 4 kV;

3) they should adapt themselves to large variations of the lampvoltages;

4) the ballasts should not destabilize the lamp arc discharge, and

5) the ballasts should be compatible with lamp characteristics, so as tomaximize the lamp's service life.

Therefore, when replacing the fluorescent lamp of FIG. 1 with an HIDlamp 4, as shown in FIG. 2, the ballast of FIG. 1 will not operate theHD lamp, for the following major reasons:

An HID lamp is not consistently susceptible to striking and is notnecessarily in a state of readiness for striking. In fact, the circuitof FIG. 1 enables a low power (70-150 W), cold HID lamp to be struck andeven brought to the operation mode. But if the lamp has operated atrated power and is shut off for some reason, the subsequent attempt toswitch on the hot lamp will prove to be unsuccessful and will damage themain components of the circuit, first of all, the power switches.

As can be seen in FIG. 2, the oscillation circuit is shorted only whenthe lamp is struck (the lamp shortens the C₂ capacitor). In all othersituations, when the lamp is not struck; the lamp is not present; thelamp is damaged; the lamp circuit is broken, etc., the oscillationcircuit is not shortened, which inevitably results in a failure of thedevice.

Therefore, the direct use of an electronic ballast intended forfluorescent lamps in HID lamp circuits is ruled out, since it isimpossible for such a ballast to provide reliable operation of an HIDlamp under actual operating conditions.

It is thus a broad object of the present invention to provide a methodfor operating HID lamps with devices built according to the basictopology of electronic ballasts for fluorescent lamps, which takes intoaccount significant physical and design features of these lamps, such astheir insusceptibility to striking and the fact that in the absence of alamp in the circuit, the series L-C circuit is not broken. The methodthus provides optimal conditions for striking, heating and operation ofHID lamps.

DISCLOSURE OF THE INVENTION

The invention provides a method for operating electronic ballasts forHigh Intensity Discharge (HID) lamps, said electronic ballasts having adriver, two power switches connected in a half-bridge arrangement, an LCseries circuit, a driver controller for controlling the operation of thedriver, a current sensor in the lamp circuit, and a power sensor in thepower switch circuit, said method comprising (a) generating pulses offrequency f₁ for a duration of time t₁ being equal to n/f₁, where n is apositive number, and f₁ equals the resonance frequency of the ballast'sLC series circuit; (b) monitoring the existence of current in the lampcircuit after the duration of time t₁ has elapsed, and in the event thatthere is no current in the lamp circuit, proceeding to step (h); (c)monitoring the current in the lamp circuit, and proceeding to step (h)upon determining that the current in the lamp circuit has ceased toflow; (d) continuing the generation of said pulses of frequency f₁ for apredetermined duration of time t₂ counting from the start of thegeneration of said pulses according to step (a); (e) switching thefrequency f₁ of said pulses to an operating frequency f₂, at which a setpower for the lamp is reached; (f) monitoring the power on the lamp andstabilizing this power at the level of the power set for the lamp, bygradually modifying the frequency f₂, and proceeding to step (h) in theevent that the power in the lamp circuit exceeds the power set for thelamp by a given margin; (g) monitoring the current in, and power of, thelamp circuit according to steps (c) and (f); (h) inhibiting thegeneration of said pulses for a predetermined duration of time exceedingt₁ and approximately equal to t₂/k, where k is a positive number; (i)proceeding to step (a) until the said predetermined duration of time t₂,counted from the start of the generation of pulses according to step(a), has elapsed; and (j) inhibiting the generation of said pulses untilthe power to the ballast is first switched off and then on.

In accordance with the invention, there is also provided a device foroperating electronic ballasts for High Intensity Discharge (HID) lamps,said electronic ballasts having a driver, a power switching circuitincluding two power switches connected in a half-bridge arrangement, andan LC series circuit, said device comprising a driver controller forcontrolling the operation of said driver, a current sensor connected ona line leading and adjacent to an electrode of the HID lamp, and a powersensor incorporated in the power switching circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with certain preferredembodiments with reference to the following illustrative figures so thatit may be more fully understood.

With specific reference now to the figures in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

FIG. 1 shows a typical circuit diagram of a prior art electronic ballastfor operating fluorescent lamps;

FIG. 2 shows the circuit diagram of FIG. 1, in which a fluorescent lampis substituted by an HID lamp;

FIG. 3 shows a device utilizing solid state ballasts for operating HIDlamps in accordance with a first embodiment of the present invention;

FIGS. 4a-4 e show waveforms of progressive cycles for ignition, warm-upand operation of an HID lamp;

FIG. 5 illustrates waveforms in the event of lamp short-circuiting;

FIG. 6 illustrates waveforms in the event of lamp circuit malfunction;

FIG. 7 is a detailed circuit diagram of the driver controller, mainlyshowing the digital part thereof;

FIG. 8 is a detailed circuit diagram of the driver controller, mainlyshowing the analogue part thereof, and

FIG. 9 shows a device utilizing solid state ballasts for operating HIDlamps in accordance with a second embodiment of the present invention;

DETAILED DESCRIPTION

Referring to FIG. 3, there is shown a circuit for igniting and operatingHID lamps utilizing solid state ballasts. In addition to the circuit'sper se known components, described above with reference to FIGS. 1 and2, the circuit also includes a driver controller 6, an induction-typecurrent sensor 8 connected in circuit on the line leading and adjacentto an electrode of the lamp, and a lamp power sensor 10 incorporated inthe power switch circuit on the common conductor. In addition, there isillustrated a power supply 12 adapted to provide the power suitable forthe specific, non-limiting, example illustrated in the drawing foroperating the electronic ballast circuit of a 400 W HID lamp.

Reference is now also made to FIGS. 4-6.

Upon the application of power from the power supply 12 to the circuit,the driver MGD produces and applies the preset required voltage andcurrent. As shown in FIG. 4, waveform I represents the driver's outputvoltage; waveform II represents the voltage on the lamp 4; and waveformIII represents the current on sensor 8.

The striking of the HID lamp, of a selected set power, is effected bygenerating pulses having a pulse frequency f₁ which equals the resonancefrequency of the ballast's LC series circuit, e.g., about 50 kHz, for aduration of time t₁=n/f₁, where n is a positive number from 3 to 10.Over the course of this duration, all electronic components of theoutput stage withstand the current spikes, which far exceed theoperation mode current. However, if the striking pulses, of a durationof n/f₁ seconds, fail to strike the lamp, pulse generation stops. Thenext attempt to strike the lamp by similar striking pulses is carriedout after a duration of time t₂/k, where k is a positive number, e.g.,within about 20 seconds, as seen in FIG. 4b. The positive numbers n andk may be constant or non-constant.

Since the longest time required for a hot HID lamp to cool down so thatit is again susceptible to striking will be about 2 minutes, the numberof striking pulses applied should be at least six (see FIGS. 4c to 4 e).

The time which passes before striking the HID lamp, i.e., the number ofgroups of pulses striking the lamp before ignition, varies in a discretemanner and depends on the state of the lamp and readiness thereof forstriking. For example, a cold lamp in good working condition is struckby the first striking pulses (FIG. 4a), and on the other hand, a hotlamp is struck by one of the subsequent striking pulses, depending onthe “warm-up level” of the lamp (FIGS. 4b-4 e). It is clear that, oncethe lamp is struck, the generation of frequency f₁ does not cease and,as soon as the initial warm-up stage is over (within about 2 minutes,counting from the first application of the first striking pulses), it isswitched to a working or operating frequency f₂, e.g., about 30 kHz, andthe lamp continues to warm up until the operation mode is reached. Thesignal confirming that the lamp ignited originates at the current sensor8, located in the lamp circuit.

An HID lamp is known to require a peak voltage of 3 to 4 kV for beingstruck by a single pulse having a duration of not less than 1microsecond. Providing a train of high voltage pulses for striking,decreases the required striking voltage of the lamp. In this particularexample, the required voltage does not exceed 3 kV.

The operation mode of the driver MOD takes into consideration all of thespecial features of HID discharge lamps, and thus reliably provides forstriking, warming up, and normal operation mode. Hence, the drivercontroller 6 governs the driver's operation and initial preset warm-upfrequency f₁. The frequency f₁ exceeds the operation frequency and isdetermined in such a way that the lamp's initial warm-up current islimited. This results in the reduction of erosion of the lamp'selectrodes and thus contributes to the increase of the lamp's servicelife. Once the lamp is ignited, the driver controller 6 controls thelamp's operation frequency f₂. Due to the feedback obtained from thepower sensor 10, the working frequency varies smoothly in such a mannerthat the illumination is maintained at a constant preset level, ordecreased to a level given by the setting of the driver controller.Hence, the power on the lamp is stabilized at the level of the power setfor a particular lamp, by gradually modifying the frequency f_(2.)

Furthermore, the driver controller 6 also governs the inhibition of thedriver's operation and in the event of a sharp increase of the loadpower, e.g., in case the lamp line short-circuits, the power sensor 10signal exceeds the rated power by a given margin and the drivercontroller 6 inhibits the driver's operation for a duration t₂/k, e.g.,for about 20 seconds, following which the driver controller 6 switchesto the initial operation cycle as illustrated in FIG. 5, wherein I isthe driver's output voltage, II is the voltage on the lamp 4, and III isthe signal of the power sensor 10.

If the cause of failure is not eliminated within the next two minutes orso, the driver controller 6 inhibits the driver's operation until thepower supply 12 is switched off and then is subsequently switched on.

Similarly, the driver controller 6 inhibits the driver's operation onreceiving a signal from current sensor 8, indicating that the lampcircuit current is stopped due to lamp line breakage, lamp failure,etc., as shown in FIG. 6, wherein I is the driver's output voltage, IIis the voltage on the lamp 4, and III is the signal of the currentsensor 8.

Referring to FIGS. 7 and 8, there is illustrated, by way of exampleonly, a possible embodiment of the controller's detailed circuitdiagram.

In general, the digital part of the driver controller (FIG. 7) sets allof the required time intervals of the lamp's ignition cycle, includingits warm-up period, controls the signal from the current sensor in thelamp circuit and produces three output signals:

1) Signal P, permitting the driver to start generation of pulses;

2) Signal f, effecting switching from frequency f₁ to operatingfrequency f₂, and

3) Signal g, causing the switching off of the circuit in the event thatno current is detected by the current sensor in the lamp circuit.

The analog part of the driver controller (FIG. 8) is responsible formaintaining the set power of the lamp, producing a reset signal in theevent that the power in the lamp circuit exceeds the set power by apredetermined margin. A light indicator 90 (FIG. 8) may optionally beprovided, that turns on when the lamp reaches the set power.

The RESET signal, required to bring the circuit to its initial state, isformed by components 18, 20 (FIG. 8) and 22 d (FIG. 7). Pulses aregenerated by oscillator/counter 24 and repeated every 30 seconds. Theduration of the pulses (100 mks) is set by monostable multivibrators 26,28. The first pulse is generated, e.g., 4 seconds after power issupplied to the circuit, by the additional trigger 30. Binary counter 32sets oscillator/counter 24 to reset after a two-minute interval, andalso forms a signal f for switching from frequency f₁ to operatingfrequency f₂. Pulses of 100 mks each are fed to the circuit activatingthe driver, consisting of resistors 34, 36, transistor 38, diode 40 andcapacitor 42, and to trigger 44. When the lamp is struck, the currentsensor 8, together with the circuit composed of the diode 46, resistor48, stabilatron 50 and capacitor 52, form a logical “one” signal thatsets the trigger 44, thereby allowing the subsequent operation of thedriver. Component 54 forms the RESET signal in the event that there isno signal from the current sensor 8 and its associate circuit. LED 16indicates that trigger 44 is brought to RESET, namely, that the circuitis in its initial state. LED 16 turns off during the lamp ignition andsubsequent normal operation.

The circuit for controlling the power includes a non-inverting amplifier56 having an amplification factor of, e.g., 11; comparator 58 forcomparing the signal from the amplifier with the voltage formed byresistors 60, 62, and inverting amplifier 64 that produces the voltagerequired for normal operation of transistor 66, using the bias circuitincluding resistors 68, 70, 72 and transistor 74. The bias voltagevaries in the event that transistor 74 is closed by signal f. Thegenerated frequency of driver MGD may vary with voltage variation at thesource of transistor 66, due to the change in the capacitance of thegate/source junction. Operational amplifier 76 forms the RESET signal inthe event of voltage at the output of amplifier 56 exceeding thereference signal formed by resistors 78, 80. The power controllingcircuit has a deep negative feedback due to capacitors 82, 84, 86. Thesensitivity threshold of comparator 58, and consequently the power onthe lamp, are controlled by potentiometer 88, while the protectionthreshold is set by potentiometer 88. LED 90 provides an indication thatthe power set for the lamp has been attained.

In the previous embodiment, the current sensor senses the current in thelamp circuit at resonant frequency f₁ after the lapse of a time periodof a duration t₁=n/f₁. When the current is insignificant, however, thisnecessitates a separate current sensor, for example, an inductancesensor, which can sense low current. Hence, in accordance with thefurther embodiment shown in FIG. 9, an intermediate frequency f₂ isintroduced and the current in the lamp circuit is sensed after the lapseof a period of time of a duration t₁+t₂, wherein t₂=m/f₂ and m is aninteger. The introduction of the frequency f₂, lower than the resonancefrequency f₁ into the working regime of the ballast, causes the currentin the lamp circuit to increase. This has made it possible to sense thecurrent in the lamp circuit with a resistance sensor, i.e., the powersensor 10 included in the circuit of the lower switch feeding the lamp4.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A method for operating electronic ballasts forHigh Intensity Discharge (HID) lamps, said electronic ballasts having adriver, two power switches connected in a half-bridge arrangement, an LCseries circuit, a driver controller for controlling the operation of thedriver, a current sensor in the lamp circuit, and a power sensor in thepower switch circuit, said method comprising: (a) generating pulses offrequency f₁ for a duration of time t₁ being equal to n/f₁, where n is apositive number, and f₁ equals the resonance frequency of the ballast'sLC series circuit; (b) monitoring the existence of current in the lampcircuit after the duration of time t₁ has elapsed, and in the event thatthere is no current in the lamp circuit, proceeding to step (h); (c)monitoring the current in the lamp circuit, and proceeding to step (h)upon determining that the current in the lamp circuit has ceased toflow; (d) continuing the generation of said pulses of frequency f₁ for apredetermined duration of time t₂ counting from the start of thegeneration of said pulses according to step (a); (e) switching thefrequency f₁ of said pulses to an operating frequency f₂, at which a setpower for the lamp is reached; (f) monitoring the power on the lamp andstabilizing this power at the level of the power set for the lamp, bygradually modifying the frequency f₂, and proceeding to step (h) in theevent that the power in the lamp circuit exceeds the power set for thelamp by a given margin; (g) monitoring the current in, and power of, thelamp circuit according to steps (c) and (f); (h) inhibiting thegeneration of said pulses for a predetermined duration of timeapproximately equal to t₂/k, where k is a positive number; (i)proceeding to step (a) until said predetermined duration of time t₂,counting from the start of the generation of pulses according to step(a), has elapsed; and (j) inhibiting the generation of said pulses untilthe power to the ballast is first switched off and then on.
 2. Themethod as claimed in claim 1, wherein n is a number from 3 to
 10. 3. Themethod as claimed in claim 1, wherein t₂ is a duration of time from 2 to15 minutes.
 4. The method as claimed in claim 1, wherein k is a numberfrom 6 to
 30. 5. The method as claimed in claim 1, wherein n isconstant.
 6. The method as claimed in claim 1, wherein n isnon-constant.
 7. The method as claimed in claim 1, wherein k isconstant.
 8. The method as claimed in claim 1, wherein k isnon-constant.
 9. A method for operating electronic ballasts for HIDlamps, said electronic ballasts having a PFC, a driver, two powerswitches connected in a half-bridge arrangement, an LC series circuit, adriver controller for controlling the operation of the driver, and apower sensor in the power switching circuit, said method comprising: (a)generating pulses of frequency f₁ for a predetermined time period t₁,being equal to n/f; where n is a positive integer and f₁ is equal to theresonance frequency of the LC series circuit of the ballast; (b)switching the frequency f₁ to frequency f₂,f₂ being lower than f₁; (c)sensing the active power in the lamp circuit after the elapse of apredetermined time period t₂ equal to m/f₂, where m is a positiveinteger, and if no active power is sensed in the lamp circuit,proceeding to step (h); (d) continuing the generation of said pulses offrequency f₂ during a predetermined time period t₃, which commences whenthe generation of pulses in step (a) is started; (e) switching thefrequency f₂ of said pulses to an operating frequency f₃, at which a setpower for the lamp is reached; (f) monitoring the power on the lamp andstabilizing said power at the power level set for the lamp by graduallymodifying operating frequency f₃; (g) monitoring active power in thelamp circuit and proceeding to step (h), provided no active power issensed; (h) discontinuing the generation of said pulses during apredetermined period of time approximately equal to t₃/k, where k is apositive integer; (i) proceeding to step (a), until said predeterminedperiod of time t₃ elapses; (j) discontinuing the generation of saidpulses for a predetermined period of time t₄; (k) repeating steps (a),(b), (c)p times, wherein p is a positive integer, provided that duringstep (c), transfer to (h) has taken place; and (l) discontinuing thegeneration of said pulses until power to the ballast is switched off andsubsequently switched on.
 10. The method according to claim 9, wherein nis an integer from 3 to
 10. 11. The method according to claim 9, whereinm is an integer from 3 to
 10. 12. The method according to claim 9,wherein k is an integer from 4 to
 30. 13. The method according to claim9, wherein p is an integer from 3 to
 5. 14. The method according toclaim 9, wherein t₃ is a time period from 2 to 5 minutes.
 15. The methodaccording to claim 9, wherein t₄ is a time period from 10 to 20 minutes.16. The method according to claim 9, wherein n is constant.
 17. Themethod according to claim 9, wherein n is non-constant.
 18. The methodaccording to claim 9, wherein k is constant.
 19. The method according toclaim 9, wherein k is non-constant.
 20. The method according to claim 9,wherein p is constant.
 21. The method according to claim 9, wherein p isnon-constant.